mixing existing copper tubing with new pex tubing

I am in the process of re-piping the hot water heating system in my house. Currently it is all 3/4" "M" tubing. I am re-piping it because the plumber before did not run the zones in proper loops and also ran loops that went up and down three stories to avoid opening up walls.

So now after half way through i am realizing that i should have used Pex in my re-pipe from the beginning. I am soldiering in very tight corners and using hundreds of fittings not to mention almost burning the house down once!!

Anyway the question is can i now (in mid stream) connect Pex adapters and go on from there in all pex tubing?? Is there some reason i am not aware of not to do this besides it will look a bit funny. I was a plumbing mechanic for about 10 years but have not worked in the plumbing trade for nearly 20 years now. I know how to solder good connections but after 40 lengths of pipe and many fittings i want a easier fast way.